Light sources having a high optical power density are key components for a multiplicity of applications. By way of example, laser diodes composed of a nitride-based compound semiconductor material system have a high market potential for projection systems, in particular those having luminous fluxes of between 1,000 and 20,000 lumens.
Therefore, components having high output powers and compact housings are required for such applications. For cost reasons, and in the context of standardization, housings of the so-called TO type series (TO: “transistor outline”) in the form of TO metal housings (“TO metal can”) are customary, for instance in the form of the known structural sizes TO38, TO56 and TO90, wherein the TO metal housings are substantially manufactured from steel. Such standard TO designs, also designated hereinafter as “TO housings” for short, are usually used nowadays for laser diodes. However, currently available laser diodes in TO housings have been limited so far to optical powers of less than 3 watts, which is insufficient for many applications. To date, however, it has not yet been possible to achieve optical powers of above 3 watts with such designs.
By way of example, C. Vierheilig, et al., Proc. SPIE, vol. 8277, 82770K, 2012, discloses blue-emitting nitride-based laser diodes in TO housings which, at room temperature in continuous wave operation, can emit light having a wavelength in the range of 440 nm to 460 nm with an output power of a maximum of 2.5 watts.
In the case of such laser diodes, the TO housings have thermal inadequacies, in particular during mounting, which is customary for technical reasons, with that side of the substrate which faces away from the semiconductor layer sequence on a heat sink between a housing and a laser diode, such that the semiconductor layer sequence is arranged at the top as seen from the housing (“Epi up”).
Alongside the standard TO housings composed of high-grade steel, TO housings are also known which, for better heat dissipation, have a base that is based on copper or has a copper core and a steel surface. However, studies have been able to show that the use of such modified TO housings alone does not lead to an increase in the output power of laser diodes.
In the case of red and infrared power laser diodes, in particular on the basis of arsenides, thermally optimized mounting concepts with very direct heat dissipation are known, in particular mounting with that side of the semiconductor layer sequence which is situated opposite the substrate downward (“Epi down”) on a heat sink between the laser diode and a housing and furthermore the use of a copper carrier instead of a TO housing.
However, such measures are unsuitable for nitride-based laser diodes since a cost-effective capping for protecting the laser against contamination and mechanical damage is not possible for a copper carrier. Particularly, for example, in the case of use in the automotive sector, moisture and chemicals can be critical and may necessitate a hermetic capping in order to protect the laser diodes from such external influences. Since, in the case of nitride-based laser diodes, the p-side is typically arranged on that side of the active region which faces away from the substrate, and is made as thin as possible since the operating voltage can increase with increasing thickness of a p-doped nitride-based semiconductor layer. Therefore, “Epi down” mounting of nitride-based laser diodes can easily lead to short circuits and thus to a reduction of the yield. This is because the active region is situated very near the ? contact in the case of nitride based laser diodes.